GAVINS POINT, S.D.
— Explorers Lewis and Clark took 14 months to navigate the Missouri River, braving sandbars, currents, and sometimes hostile Indians. But in retrospect, they may have had it easy. Riding the nation's longest river doesn't compare with the challenge of living by it.

Despite the nation's largest system of dams, which provide power, water, and recreation to millions of people, the Missouri remains unpredictable. Upstream communities worry increasingly about drought. Downstream farmers fear floods. The river is silting up its own lakes and, ironically, killing some of its own native wildlife.

Thus, the Missouri is turning into a giant tug of war over a central question: Should the United States, which has proved so adept at taming rivers, now take a step or two toward untaming the Missouri?

The nation is taking such steps on smaller river systems, but this 2,341-mile ribbon of water runs through so many lives in so many ways that compromise has proved impossible so far. The US Army Corps of Engineers, which regulates the river's flow, has postponed  again  announcing its preferred solution.

Nevertheless, some compromise seems essential if the nation is to adjust wisely to changing conditions. Otherwise, one of the nation's most endangered rivers and extensive ecosystems will degrade further. And upstream communities, facing an increased thirst for water, may struggle mightily the next time drought hits.

If controversies can have epicenters, this one probably lies here at Gavins Point Dam, some five miles west of Yankton, S.D. The last of six major dams on the Missouri, it lies within an hour's drive of all three of the river's ecosystems. More important, it houses the effort to save one of the river's most troubled native inhabitants: the pallid sturgeon.

With tiny eyes and shovel-shaped snouts, it is a native of the Missouri whose ancestors date back to the time of the dinosaurs. Reaching into one of 30 tanks that look like miniature swimming pools, Herb Bollig nets two young pallid sturgeon. These eight-inch fish, being raised here at the National Fish Hatchery, which Mr. Bollig manages, represent the future of the species.

The problem: These ancient fish are facing extinction, because changes in the river have altered their habitat.

For example, although some 1,000 to 3,000 of the old fish survive on the river, they haven't spawned since the dams were finished in the late 1960s. One reason: The dams have eliminated the natural spring and summer rises in the river, which scour the shores of vegetation and clean the rock and sand that sturgeon eggs can adhere to.

Also, because lakes have built up behind them, the dams release colder-than-normal water from the bottom of the lakes. Water below 60 degrees not only discourages the fish from spawning in the river, but also causes them to stop eating. To their dismay, biologists discovered adult sturgeon starving in their own native habitat.

Many species on the decline

The endangered sturgeon  plus two endangered birds  are only the leading edge of a general decline of wildlife along the Missouri. According to a National Research Council report earlier this year, 51 of the 67 species native to the river are listed as rare, uncommon, or decreasing in numbers.

"Do we want to continue down the road of rack and ruin?" asks Bollig, standing in his fish hatchery. "Are we going to stand by and let all these species decline? Or do we draw the line and say: 'Let's do something now!' " Although the efforts of hatchery officials have probably staved off immediate disaster, the pallid sturgeon population won't be secure until it has suitable habitat again to spawn in the wild.

For a dozen years, the US Army Corps of Engineers has looked for solutions. In the latest twist, the corps proposed six possibilities: the status quo, a water-conservation measure that would change flows upstream, and four proposals that would alter releases from Gavins Point, which would mimic somewhat the old river's natural spring rise and summer decline.

When the corps solicited public comments, it was deluged. Of 55,000 comments, 54,000 called for some kind of change.

Unfortunately, there's little agreement on exactly what to change. Upstream communities, shocked by a long drought in the late 1980s and early '90s, want to retain more water in the reservoirs during dry periods. Barge operators downstream prefer the status quo, which calls for the corps to release enough water to maintain a barge channel on the lower Missouri. Lake recreationists also want to keep water in the reservoirs. River enthusiasts, however, side with the barge operators. Farmers and environmentalists both call for wise stewardship of the shoreline, but their solutions differ wildly.

"I've come to the conclusion that there is a constituency for and against all the features of all the plans," says Paul Johnston, chief of public affairs for the corps' northwestern division in Omaha, Neb. Earlier this month, the corps announced it had postponed indefinitely the announcement of its preferred solution, pending further talks with the US Fish and Wildlife Service.

The morphing Missouri

Part of the challenge stems from the river basin's varied geography. Gushing from its source high in the Rocky Mountains of western Montana, the river drains one-sixth of the nation's landmass, and it flows through regions that get 10 inches of rain a year and others that get 40. It also morphs between natural river and reservoir until the last third of its run, when it turns into a shipping canal that dumps into the Mississippi River just north of St. Louis.

Even in normal times, managing a single river basin to the satisfaction of 10 states, 28 native American tribes, and 10 million people can be more than a little tricky. States have already filed suit this year to keep the corps from manipulating short-term water flows. The prospect of updating the Master Manual, which has guided the releases from the Missouri's dams for 60 years, has corps officials stymied.

"They're in a tough spot," says Joe Citta, environmental-services manager for the Nebraska Public Power District. The utility, which operates a nuclear plant on the river south of Brownville, Neb., has its own worries. A decline in summer flows would likely raise the river's temperature, possibly forcing the plant to shut down on the warmest summer days. For various reasons, in fact, the plant can't take in or discharge water it uses for cooling when the river temperature reaches 90 degrees.

Ripple effect

Thus, on the warmest days, when power is needed the most, the statewide utility might have to shut down 20 percent of its electric generation. Moreover, if the river flow drops, there is no guarantee of enough water for barge traffic during the summer.

Farther south, in Rulo, Neb., farmer Stephen Johansen balances an envelope in his hand. "If you hold this envelope as flat as you can, it still has more slant than the river," he says. "Your kitchen table has more slant than the Missouri River."

Because this section of the lower Missouri drops less than a foot for every mile it runs downstream, it meanders in its natural state. When Mr. Johansen's father-in-law was a boy, he woke up one morning to find the river had shifted thousands of feet overnight and was suddenly running next to the house where he was staying.

Over time, the river has stranded river cities and flooded them. It's also added to farmers' land, only to slice it away from those on the other side of the bank.

The dams, plus the channelization of the river from Sioux City to St. Louis, have alleviated such problems. Even so, there's little margin for error.

"The plan is a disaster," says Johansen of the corps' proposals to increase the spring rise along this section of the river anywhere from 3.3 feet to 4.3 feet about once every three years. "If they raise the river three feet, we will never raise a crop." Even under current conditions, he's rented land he hasn't been able to harvest because of flooding during a recent five-year wet cycle.

Economically, changing the flows would boost the river's benefits slightly, according to corps estimates. Hydropower, which alone provides 40 percent of the river's total $1.9 billion in annual benefits, would see a small positive increase. Water supply, the No. 2 use, would be unaffected, and flood control, No. 3, would experience a small decrease.

The bigger effects lie with the less important economic uses. Navigation  primarily barge traffic  would take the biggest hit: benefits reduced by a quarter to a third. Then again, its economic impact is minuscule, accounting for less than half of 1 percent of the river's benefits. Recreation, which accounts for nearly 5 percent of benefits, would see a 2 to 5 percent gain.

"I bet you can count on one hand ... the number of barges at Sioux City," says Kerry Kasulka, who lives on his houseboat here on Lewis and Clark Lake during the summer while managing a South Dakota grain elevator.

He adds that the more important need is for consistent levels on the reservoirs, especially since the upper end of the lake is filling with silt. "Thirty years ago, you could run to Springfield [S.D.] in 30 minutes. You can't even hardly get to Springfield anymore" because of the silt.

Unusual political allies

Politically, the controversy cuts across party lines. It puts the leading Senate Democrat, majority leader Tom Daschle (D) of South Dakota, in a camp with six Republican governors upstream calling for flow changes.

Meanwhile, the leading House Democrat, minority leader Richard Gephardt (D) of Missouri, has joined forces with that state's Republican and Democratic senators, as well as with Democratic Gov. Bob Holden, in opposing changes.

The corps' indefinite delay in announcing its preferred solution represents a temporary victory for the latter group.

Like many reservations along the Missouri, the three tribes also depend on revenues from lake and river access leases and fishing licenses. When water levels drop, as they have along the reservoir formed by the Garrison Dam, those revenues drop, too, Mr. Hall says. Lower lake levels also expose more Indian artifacts, which has encouraged looting.

Whatever the corps eventually decides, most observers expect the matter to end up in court. But perhaps surprisingly, compensation for losses downstream could go a long way toward finding a solution. Farmers, such as Johansen in Rulo, say they would accept more flooding if the government made up their losses. Power plants that install cooling towers would not have to shut down when river temperatures got too high.

The payments might prove expensive. But then so do the costs of doing nothing.